1. Field of the Invention
The present invention relates to a Sagnac effect optical gyrometer, whose physical principle is based on a relativistic effect discovered by the physicist Sagnac in 1913.
2. Description of Related Prior Art
Optical gyrometers are well known and a description is e.g. provided on pp 91 ff of Journal of Light Wave Technology, vol. Lt. 2, No. 2, April 1984.
Details will be given of the known theory and the main evolutions of such equipments with reference to the attached FIGS. 1a and 2a.
The actual probe of a Sagnac effect optical gyrometer has a ring or turn 1 traversed by light and whose two ends have a common section 2, which respectively connects them to a light source 3 and to an interference detector 4. When the light from the source 3 is injected through the common path 2 in the ring 1, said geometrical arrangement makes it possible to bring about a division thereof into two separate light waves, whereof the first "a", passes through the ring 1 in the clockwise direction and the second "b" traverses said same ring in the counterclockwise direction, the two waves being by definition in phase because they come from the same source 3. On leaving the system, the detector 4 makes it possible to observe any interference fringes resulting from the combintion of waves a and b. By design, and in the absence of any other action, the interferences between the waves a and the waves b are perfectly constructive and correspond to a strictly zero phase shift .DELTA..rho.=0.
However, if the device rotates with an angular velocity .OMEGA. with respect to an inertial reference frame, the wave passing through the ring in the rotation direction acquires a phase lead .rho. during its passage, whereas the other acquires a phase lag -.DELTA..rho.. Thus, at the exit there is a total phase shift of 2.DELTA..rho., which modifies the interference patent read by the detector 4. As the theory of the apparatus shows that the value of the phase shift .DELTA..rho. is proportional to the vector product A.LAMBDA..OMEGA. in which A is a vector, whose magnitude is proportional to the surface of the ring and whose direction is perpendicular to the plane of said same ring and .OMEGA. is the vector of the angular rotation which it is wished to measure, it is clear that the measurement of the total phase shift .DELTA..rho. performed by the study of the modification of the said interference pattern makes it possible to obtain the value of the sought rotation .OMEGA..
Devices for measuring an absolute rotation designed on the basis of this principle have been developed, particularly since the appearance of on the one hand laser light sources and on the other optical light conducting fibers.
As the sensitivity of a Sagnac optical gyrometer is proportional to the length of the path of the light wave, the aim has naturally been to lengthen said path, which was particularly easy with optical fibers and in this way it was possible to produce gyrometers, whose detecting probe was formed from a coil of optical fibers in several turns. More recently, consideration has also been given to a construction of optical gyrometers by making use of integrated optics and for artificially increasing the length of the single turn constituted by a light microguide, use has been made of the resonant gyrometer, whereof an example is diagrammatically shown in FIG. 2a.
FIG. 2a shows the resonant ring 1 constituted by an optical microguide tangentially supplied by two light guides 5, 6, each having an entrance face 7 and an exit face 8, the light guides 5 and 6 being coupled to the resonant ring 1 with the aid of two optical couplers 9, 10. The principle of this known apparatus consists of increasing the path of the two light waves, whilst permitting the light to pass through the ring several times and examining the resulting interferences of the state of the two waves after several passages of this type. The performance characteristics of such an apparatus are consequently dependent on the number of turns which can be traversed by the light waves without excessive attenuation thereof due to propagation losses. The disadvantages of this apparatus are due to the need for the use as the light source of a laser having a considerable coherence length, which involves the use of a longitudinal monomode laser able to emit light in this form, within a wide temperature range for certain special applications of the apparatus. The construction is already difficult. Moreover, the principle of the apparatus makes it necessary to be able to accurately monitor the wavelength .lambda..sub.o of the light, which corresponds to the resonance in the ring and this is not easy to carry out using simple electronic treatments, because the aforementioned wavelength .lambda..sub.o can evolve as a function of different parameters.
It can therefore be said that although fibre optic gyrometers have arrived at a satisfactory sensitivity, the overall dimensions of the coil are prohibitive for numerous applications. With regards to integrated optics gyrometers, the need to make them operate in accordance with a resonant mode leads to very real difficulties in their practical implementation.
For the satisfactory understanding of the remainder of the present text, it is also necessary to develop a number of considerations on a useful accessory known since 1980 and which, in optical gyrometers, makes it possible to obtain operating conditions corresponding to their maximum sensitivity.
Thus, optical gyrometers usually have a completely symmetrical structure with respect to the ring traversed by the light. This is certainly the case with integrated circuit gyrometers and consequently in the absence of any rotary component, the phase shift between the rotating and contrarotating waves is zero. Therefore these apparatuses are two wave interferometers and the light intensity I collected at the exit of the passage in the optical ring is in the form EQU I=I.sub.o cos.sup.2 .DELTA..rho./2
in which .DELTA..rho. is the phase shift between the two waves.
If as a result of a rotation imposed on the apparatus and which it is wished to accurately measure, the expression .DELTA..rho. varies by .delta..rho., it is possible to write: ##EQU1##
This shows that when the phase shift between the two waves .DELTA..rho. is zero, this also applies with respect to dI and the sensitivity dI/d.rho..
However, dI/d.rho. is at a maximum if .DELTA..rho.=.pi./2, dI/d precisely representing the sensitivity of the apparatus, because it is the output intensity variation level related to the phase shift variation level.
It can be gathered from the above remarks that it is highly desirable for obtaining the optimum sensitivity of the apparatus to delay one of the waves by .pi./2 compared with the other.
In order to solve this problem, it is conventional practice to make use of the fact that light does not have an infinite velocity and takes a time T to pass through the optical ring. By placing a phase modulator in an asymmetrical position on the ring, it is consequently possible to act differently on the two waves passing through it in opposite directions and to delay one by .DELTA..rho. compared with the other. For this purpose it is merely necessary to place the modulator at the entrance for one wave, which corresponds to the exit for the other. If the modulator frequency is well calculated, i.e. if it is f=2/.tau., it can be demonstrated that it is possible to perfectly produce a non-reciprocal effect in the light duct, which leads to the following advantages:
1) the readings are made by positioning at the maximum sensitivity point of the apparatus for the lowest possible modulation amplitude value; PA0 2) the response of the system has a term at the modulation frequency f proportional to sin .delta..rho., (.delta..rho. being the phase variation introduced by the rotation).
It is consequently merely necessary to perform a synchronous detection of the signal at the frequency f to obtain a reading of the apparatus under excellent signal-to-noise ratio conditions.
The above considerations concerning the interest of a modulator placed on the light ring and which have been usefully given before providing the following specific description are well known and this is confirmed by Journal of Light Wave Technology, vol. LT, No. 2, April 1984, referred to hereinbefore, as well as the article "All-single-mode fibre-optic gyroscope with long-term stability", published on p 502 ff of Optics Letters, vol. 6, No. 10, October 1981.
Such modulators, optionally produced in integrated optics with an electrooptical material have been described in the article "An overview of fibre-optic gyroscopes", Journal of Light Wave Technology, vol. LT2, No. 2, April 1984, pp 91 to 107.
The aforementioned phase shift function of one of the "rotating" waves compared with the other "counterrotating" waves has been brought about by a tricoupler placed at the entrance of said spiral and which is much simpler to produce than the modulator described hereinbefore. Such constructions are, however, limited to fiber gyrometers for which pronounced miniaturization is impossible.
If it is wished to obtain a reliable and robust miniaturized gyrometer at a reduced price as a result of mass production, the solution of optical integration is the only one which can be envisaged at present.
This is the way taken by the Japanese document JP-A-62 247 209 published on 28.10.1987 and which effectively describes an optical gyrometer probe, whose wave guide is an integrated spiral in the form of helical path on a substrate. At the entrance of said spiral there are a laser diode serving as the light source and a polarizer, whilst at the exit a detector is provided. A modulator is positioned on the spiral in the vicinity of one end. However, in said construction, only the actual waveguide constituting the spiral is integrated onto the substrate, which limits the miniaturization possibilities for the apparatus and its mass production possibilities.
With regards to the prior art in connection with integrated optics gyrometers and spiral probes, reference can more particularly be made to the following documents:
EP-A-475 013, published subsequent to the French priority date of the present application, reveals the association of an integrated structure tricoupler and a spiral probe, apparently produced by non-integrated, optical fibers.
Japanese patent JP-A-10-064 283 reveals a gyrometer having a probe in the form of an integrated spiral with a direct intersection of the waveguides of the spiral.
Japanese patent JP-A-62-247 209 describes an integrated spiral gyrometer, but in which the intersection structure of the guides of the spiral is not integrated.